20130121-22
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Merging two database records
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Lino applications can now add a "Merge" action to any model.
:mod:`lino.core.merge`

A typical use case are duplicate partners: 
a user realizes that due to human mistake
a given company exists twice in their database,
and each of them has certain number of events, invoices, contracts 
or other related things.

Clicking the "Merge" action on a Partner will 
ask "Merge this into" (select another Partner), and if you confirm, 
Lino will move all related data from this Partner to the other one,
then delete the first one.
Or in other words: if you discover a duplicate record, select the 
one that is "too much" (usually the one with the bigger id), then 
run the "Merge" action.

Technical challenges:

- this action also merges related objects linked through a generic foreign key.
- Since I wanted a generic solution 
  it took me some time to figure out how to make Lino support
  ForeignKey to self for the `other` field of the 
  action parameter dialog.
- "hard references" (e.g. contracts) are never deleted, but unconditionally 
  moved to the target object. But for certain tables this can be disturbing: 
  That's why "soft references" (those who are lited in 
  :attr:`allow_cascaded_delete <lino.core.model.Model.allow_cascaded_delete>`)
  may optionally be automatically deleted.
  The corresponding checkbox fields depend of course on the model,
  so this action is a first example of an automaically generated Layout.
  
I also used this occasion to make more intensive usage of 
`Django's Signals <https://docs.djangoproject.com/en/5.0/topics/signals/>`__,
a feature which I have been ignoring for far too long.
:mod:`lino.core.signals`

The change also caused a few internal optimizations:

- Lino no longer uses copy.deepcopy to create copies of each action.
  (I guess I introduced this before BoundAction existed...)
  
- :class:`lino.ui.base.UI` is now a new-style class because I 
  override `get_patterns` and want to call super().

- `AttributeError: 'WSGIRequest' object has no attribute 'raw_post_data'`
  This was of course due to :djangoticket:`17323`.
  That's nice and no problem for me since Lino doesn't need to run on Djangos before 1.4.2.
  
- When settings.DEBUG was off, then error messages didnt work because 
  I had converted the error templates to Jinja, but Django's TEMPLATE_LOADERS 
  wasn't correctly set (and :class:`lino.core.web.Loader` had never been 
  tested).
  This caused TemplateSyntaxErrors like "Could not parse the remainder: '('bootstrap','css','bootstrap.css')' from 'site.build_media_url('bootstrap','css','bootstrap.css')'"